We propose to participate as a collaborating clinical unit in a feasibility study and phase III trial of low-fat diet to prevent breast cancer in women at increased risk of that disease. We have assembled a team of investigators with considerable experience in areas relevant to breast cancer and the trial, including community screening for breast cancer, epidemiology, biometry, nutrition counseling and behavior and participation in long-term clinical and public health trials. Depending on protocol design, we propose to randomize 400 subjects recruited from participants in the Iowa Breast Cancer Detection and Demonstration Project and women screened privately for breast cancer using similar techniques in the clinical practice of the principal investigator, a pool that numbers approximately 18,000. After protocol design and finalization, dietitians at the Des Moines Clinical Center would be trained in nutrition counseling, surveillance and adherence by the nutrition scientists at the University of Iowa. Then, those subjects randomized to the intervention group would receive a staged 20%-of-calories-as-fat diet. All subjects would be screened annually for breast cancer by palpation and mammography for at least five years. Ancillary surveillance for breast lesions can be performed by the statewide cancer incidence register (SEER program). General design and technical issues are discussed within the proposal.